This invention is generally directed to a novel resettable protector unit which can be used to retrofit terminal blocks in telecommunications circuits. More particularly, the invention contemplates a novel protector unit which can be reset for multiple uses and also includes a back-up, fail-safe assembly to prevent thermal runaway.
Protector units are used in central offices to provide protection to personnel and equipment from abnormal conditions, such as over-voltages and over-current, induced on telephone lines. A telephone line generally includes a pair of conductors which are referred to as tip and ring conductors. The incoming line pairs are connected to the tip and ring conductors of the central office at a connector block. The protector units are installed in the connector block so as to provide protection for the line from an over-voltage condition and an over-current condition for the incoming line pairs. Generally, a separate protector unit is used for each of the tip and ring conductors of each line pair. The protector units are placed between each tip and ring conductor and a ground circuit.
A brief over-voltage condition generally will cause a temporary grounding of the incoming line until the over-voltage condition has passed. An over-current condition generally causes a permanent grounding of the incoming line. In this case, the protector unit must be removed and replaced on the line which has experienced the over-current condition.
Present current limiters or heat coils, such as the 76A type heat coil, for central offices with model connectors and blocks with protection, such as the C-50 and the 300 Type, are non-resettable when the current limiter or heat coil experiences an over-current condition. These current limiters or heat coils typically incorporate a soldered connection wrapped with resistive wire. When a high sustained current, usually greater than 500 milliamps, flows through the wire, heat is generated and transferred to the soldered connection which melts and allows movement to occur. This movement allows a one-time switching action to occur which grounds the incoming line to protect the telecommunications lines against the over-current. After this one-time switch, the current limiter or heat coil must be disposed of, and replaced, even after the over-current has subsided, or is removed.
The novel protector unit of the present invention is designed to retrofit into existing applications, replacing the 76A heat coil in C-50 connection frames and in 300-Type protectors and other older connection product. The novel protector unit of the present in invention is resettable and incorporates a back-up, fail-safe assembly to prevent thermal runaway. Other features and advantages will become apparent upon a reading of the attached specification and upon a study of the drawings.